1. Technical Field
The invention relates to an image capturing apparatus such as a digital video camera or a digital still camera and a face area extraction method and in particular to an image capturing apparatus and a face area extraction method for making it possible to enhance the detection accuracy of a face area in a captured image.
2. Description of the Related Art
A recent image capturing apparatus has installed functions of automatically performing focus processing of focusing on the face of a person and automatically performing exposure adjustment processing and white balance correction processing so as to capture the skin color of the face finely when a person on the background of a landscape is a subject.
For example, with a digital still camera, before a still image is captured, image capturing data input subsequently in a moving image state from a solid-state image capturing element is analyzed, thereby extracting the area where a face exists, and with the face area in focus, exposure adjustment and white balance correction are automatically executed so as to take a picture of the skin color of the face fine.
To extract a face area in a captured image, a pattern matching method and a skin color detection method are available.
The pattern matching method is a method wherein various face patterns such as a pattern of viewing the face of a human being from the front, a pattern of viewing the face of a human being from a side, a face pattern of an adjust, and a face pattern of an infant are retained in memory in an image capturing apparatus and are used to extract the image area matching any of the face patterns from the captured image according to pattern matching.
The skin color detection method is a method wherein a captured image of one screen is divided into 8×8=64 areas, for example, the hue in each division area is found based on the integration data of R (red) G (green) B (blue) for each division area, and it is determined that a face exists in the division area where the hue is a skin color.
To extract a face area, the face area needs to be extracted with good accuracy. If a face area is erroneously extracted, focus processing is performed by assuming that the portion which is not essentially a face is a face and therefore an image with the subject person out of focus (so-called out-of-focus picture) is captured and an exposure failure or a white balance failure occurs, resulting in unnatural color of the face of the subject person.
For example, in the pattern matching method, a leaf pattern in the background may match a face pattern depending on the condition of a light beam, leading to erroneous extraction. In the skin color detection method, an essential person face may be erroneously determined a face other than a person face as the lipstick color or the spectacle color confuses the determination or if a background image of a color resembling the skin color exists, it may be erroneously extracted as a face.
JP 2001-119622 A, JP 2003-107555 A (corresponding to US 2003/071908 A), and JP 2005-318554 A (corresponding to US 2005/231628 A) relate to the face area extraction methods:
To extract a face area according to the pattern matching method, if the number of the types of face pattern templates provided in an image capturing apparatus is increased and matching processing is performed while the relative enlargement ratio or reduction ratio to the image to be matched with each face pattern used in pattern matching is changed finely, the face area detection accuracy is improved. However, to do this, the computation processing load grows and a high-performance and expensive processor becomes necessary to perform matching processing in a short time.
To extract a face area according to the skin color detection method, the hue of each of division areas further smaller than 8×8=64 areas described above is found, whereby the face area detection accuracy is improved. However, to do this, as mentioned above, the computation processing load also grows and a high-performance and expensive processor becomes necessary to extract a face area in a short time.